Although this report encompasses only 10 months of work and though several results are still of preliminary nature, it seems possible to draw the following tentative conclusions: 1) Several results confirm that TWR 1339 is active in reducing lung metastasis in the model of Lewis lung carcinoma transplanted intramuscularly. More specific knowledge is now available in regards to variations in experimental conditions of treatment to observe activity. It is also confirmed that TRW 1339, at variance with classic antitumor agents (e.g.cyclophosphamide) is not effective on the primary tumor in conditions where activity on metastasis formation is observed. 2) As a result of testing several compounds, 6 additional polymers with antimetastatic activity have been found, 3 among these exhibiting remarkable potency in more than one experimental system. On the other hand, of a large series of TRW 1339 congeners tested, none was found to show antimetastatic activity in vivo. The results obtained with these substances of different physico-chemical characteristics form a basis for rationalized efforts already under way to determine the properties of greater relevance for antimetastatic activity and for both developing further compounds with hopefully greater potency and for shedding light on the mechanism(s) of tumor dissemination. 3) Efforts aimed at elucidating the possible mode of action of these antimetastatic substance have established that TRW 1339 as well as two newly developed polymers (G4 and G13) are capable of increasing the phagocytic activity of the reticuloendothelial system, which is depressed in the presence of a disseminating tumor. In contrast to a marked effect on the RES, TRW 1339 was not shown to modify host immune responsiveness to a standard antigenic stimulus. Furthermore, the abrogation of the host immune capacity by a treatment with a potent and selective immunosuppressant, antilymphocytic serum, does not affect TRW 1339 antimetastaic activity. 4) Studies aimed to elucidate at the cell level the mechanism of action of TRW 1339 and other active polymers have been performed. TRW 1339 does not affect the in vitro electrophoretic mobility of cancer cells (K.B. line) and hence the surface electrical charge, though it showed clear signs of affecting membrane properties when time-lapse cinematography was used. While TRW 1339 increases the ad (Text Truncated - Exceeds Capacity)